The present invention relates to analog to digital conversion of small signals in the presence of a large DC offset. AD converters are used in integrated circuits for a wide variety of applications. Often, the signal to be converted contains a small AC signal that sits atop of a DC offset. The DC offset interferes in the conversion. Consequently, AD converters attempt to remove the DC offset before performing the analog to digital conversion.
Prior solutions to remove DC offsets include AC coupling. For AC coupling, a buffer or amplifier with a high pass transfer function is provided before the converter itself. The AC coupling is designed to strip off the DC component of an input signal but allows the AC component to pass to the ADC. The high pass AC coupling is usually provided in the form a large capacitor. However, large capacitors are difficult to manufacture as an integrated circuit because they take up an overwhelming amount of silicon area. Sometimes, capacitors cannot be provided within the integrated circuit and must be provided as an external capacitor, which increases complexity and cost of the analog-to-digital conversion systems.
Accordingly, there is a need in the art for analog to digital conversion of small signals that accurately removes a DC offset while at the same time minimizing cost and components.